Coal tar is one of the liquids derived from coal. Low temperature coal tar is a by-product from semi-coke generation or low temperature gasification using low rank coal (young coal), such as lignite, as feed. Coal derived liquid can also be generated from direct coal liquefaction. Liquid generated from young coal in China usually contains high level of oxygenates. It has been reported that low and mid-temperature coal tars in China contain about 20-30% phenols. The phenols in the coal derived liquid are mixture of many kinds of phenols including single ring phenols such as phenol, cresols, xylenols, longer alkyl or multi-alkyl phenols, benzenediols, and alkyl benzenediols, as well as multi-ring phenols, such as naphthols and alkyl naphthols.
Because of their extremely complicated compositions, most coal tars have been limited to being used as heating fuel, a low value application. In addition, burning coal tars without treatment has created a lot of environmental issues in many areas of China. Recently, hydroprocessing coal tars through hydrogenation to make diesel, gasoline, and other chemicals such as aromatics has become more and more popular in China. Since the concentration of phenols in coal tar is quite high, it is desirable to remove the phenols before the coal tar liquid is processed in a hydro-processing unit. Otherwise, the oxygen in the phenols will consume hydrogen (to make water) in the hydro-processing unit increasing the hydrogen consumption, which increases the cost of hydro-processing. Formation of large amount of water can also be detrimental to the hydroprocessing catalysts. Additionally, phenols generally have higher value than fuels.
Multiple technologies have been developed to recover phenols from coal tar. The traditional technology is washing the coal tar liquid with a base followed by an acid neutralization to recover phenols because phenols are acidic. The phenols are easily recovered, but separating them to obtain individual pure products is difficult. In addition, although there is a demand for phenol itself, the demand for alkyl phenols is quite low.
Therefore, there is a need for a process for treating coal derived liquids to recover and process phenols from the coal derived liquids.